The present invention relates to a new and improved construction of a gas sensing unit for use in environments containing explosive gases, which is operatively connected with a central station to thereby form a gas sensing signaling system. The gas sensing unit contains a gas sensor which, when exposed to the action of reduction gases, alters its electrical resistance.
In order to protect industrial plants or installations, pipe conduit channels, chemical storage areas and so forth against fires and also human beings from exposure to toxic gases, it is desirable to be able to detect at an incipient stage dangerous concentrations of combustible or noxious gases. In response to such detection it is then possible to initiate suitable counter measures, for instance shutting down operating installations which are improperly functioning, closing off leaking pipe conduits, starting ventilators or other exhaust apparatus, opening emergency exits and otherwise signaling to the occupants or personnel the need to leave the area and so forth, and in this way avoiding fires, explosions, toxic effects to the occupants or personnel and other damage.
In order to detect undesired and dangerous concentrations of reducing, i.e. oxidizable or combustible gases, there are known to the art gas sensing signaling or alarm systems composed of gas sensing units which are connected with a central station.
Also known to the art are gas warning equipment containing a diffusion measuring head equipped with active and inactive gas trace elements arranged in a mechanically easily detachable gas trace element insert below a cover plate formed of a gas pervious sintered or powdered metal. These gas warning devices generally use a gas detector-bridge circuit, wherein two electrically heated thermoelements are appropriately connected in circuit with one another, one thermoelement constituting a catalytically active thermoelement and the other an inactive thermoelement.
There are also known to the art gas warning devices containing as the gas sensor or gas sensing element metal oxide semiconductors, which when exposed to the action of reducing gases alter their electrical resistance. This resistance change is converted in an evaluation circuit into an electrical signal, by means of which there is activated the alarm device and there are initiated counter measures.
Representative gas sensing units are disclosed in U.S. Pat. No. 3,245,067, German Pat. No. 2,625,891 and the prior art referred to in the aforementioned copending U.S. application Ser. No. 054,786.
Since the working temperature of the gas sensors or gas sensing elements usually is in a temperature range where there has already been vastly exceeded the ignition temperature of the gases to be detected--for instance 180.degree. C. for ether and acetaldehyde, 220.degree. to 300.degree. C. for gasoline and diesel fuels, 305.degree. C. for acetylene and so forth--, it is necessary that measures be carried out which prevent that the gas warning devices cause ignition of the gases against whose presence there must be warned. For this purpose proposals have already been made to provide the gas sensors or gas sensing elements with a flame barrier formed of a small wire mesh or a grid. These flame barriers are suitable as a firedamp protection, but however provide no effective protection against explosions.
Furthermore, it has been proposed to arrange, within the region or area to be monitored, only the diffusion measuring head which is designed so as to be protected against explosions and to separately mount in relation thereto the evaluation circuit.
The utilization of semiconductors as gas sensors or gas sensing elements in gas warning devices is relatively new. The first usable semiconductor gas sensors became known to the art only at the start of the last decade, and significant in this respect are, by way of example, German Patent Publication Nos. 2,005,497 and 2,016,388. An appreciable drawback of the gas sensing elements or gas sensors which have been employed resides in the fact that, within a production or manufacturing series, there prevail great sensitivity differences, i.e. the resistance change as a function of the gas concentration is not the same for the individual gas sensors. In order to attain a positive giving or sounding of an alarm, it is therefore necessary that the evaluation circuit contains circuit components which enable balancing or compensating the aforementioned differences.
Since, on the other hand, the longevity or service life of the gas sensors or gas sensing elements is limited, it is necessary that they be checked at certain time intervals and, as the situation requires, exchanged. Consequently, the balancing or compensation operation needed of the gas sensors must be performed at the site of erection or employment. To this end the gas sensor must be brought into contact with an atmosphere containing a certain concentration of testing gas and humidity, whereas at the same time the evaluation circuit must be balanced. Since the evaluation circuit for the individual gas sensing units is located at the central station, the balancing operation for the individual gas sensors or gas sensing elements must be undertaken at this location, which, in turn, is associated with appreciable drawbacks as will be readily evident. This technique generally requires a second maintenance or service operator; with great distances between the gas sensing unit and the central station their prevail additional difficulties in communication between these two locations.
A further appreciable limitation of the heretofore known gas warning devices resides in the fact that the sensors must be accommmodated, at the site of erection or use, by means of a calibration gas to the electronic system. The calibration gases which can be obtained commercially are only present in a dry state in the transport flasks or containers for physical reasons. If, however, a dry gas is used for calibration purposes, then the sensors are appreciably less sensitive. To accomplish a correct balancing operation it is therefore necessary to moisten the test gas, so that there can be established conditions which are as close as possible to those encountered in practice. A really correct balancing, with reproducible conditions, is only therefore possible at the manufacturing plant. Yet, adjustment or balancing at the manufacturing plant was not at all possible with the heretofore prior art constructions of gas warning equipment, since the length of the lines between the gas sensor or gas sensing element and the central station is not exactly known, but however must be taken into account during balancing of the gas warning equipment.
It would have been obvious to accommodate both the evaluation circuit and the gas sensor within the same housing, i.e. to provide a gas sensing unit containing both the gas sensing element and evaluation circuit, and to bring the entire gas sensing unit to the factory in order to check and exchange, when necessary, the gas sensors or gas sensing elements. However, this likewise is associated with appreciable disadvantages. Due to the requirement of explosion safety the gas sensing units would be relatively large in design and cumbersome to handle. Additionally, the explosion-proof property of the gas sensing unit would be extremely difficult to realize and check. The evaluation circuit would be exposed to the ambient air, which would not be a tolerable state when the atmosphere is corrosive. With this type of construction the gas sensor only could be exchanged when the evaluation circuit is exposed or opened, so that there would prevail the danger of damaging and misadjustment thereof. Already with the slightest damage there no longer would have been correct the permissible tolerances for the explosion gaps, with the result that the explosion protection would no longer be present.
A further drawback would reside in the fact that explosion protected, i.e. compression-proof chambers or spaces can only be opened with difficulty, and these difficulties increase the larger the size of such chambers or spaces. An appreciable shortcoming also resides in the fact that the exchange of the gas sensing unit at its erection site is quite cumbersome, since it is also necessary that the electrical connections of the gas sensing unit with the line network be carried out in an explosion protected design, i.e. would be difficult to solve.